RxJava + retrofit, get a List and add extra info for each item

Question

I'm playing around with RXJava, retrofit in Android. I'm trying to accomplish the following:

I need to poll periodically a call that give me a Observable> (From here I could did it)

Once I get this list I want to iterate in each Delivery and call another methods that will give me the ETA (so just more info) I want to attach this new info into the delivery and give back the full list with the extra information attached to each item.

I know how to do that without rxjava once I get the list, but I would like to practice.

This is my code so far:

pollDeliveries = Observable.interval(POLLING_INTERVAL, TimeUnit.SECONDS, Schedulers.from(AsyncTask.THREAD_POOL_EXECUTOR))
            .map(tick -> RestClient.getInstance().getApiService().getDeliveries())
                    .doOnError(err -> Log.e("MPB", "Error retrieving messages" + err))
                    .retry()
                    .subscribe(deliveries -> {
                        MainApp.getEventBus().postSticky(deliveries);
                    });

This is giving me a list of deliveries. Now I would like to accomplish the second part.

Hope I been enough clear. Thanks

Answer 1

Finally I found a nice way to do it.

private void startPolling() {
    pollDeliveries = Observable.interval(POLLING_INTERVAL, TimeUnit.SECONDS, Schedulers.from(AsyncTask.THREAD_POOL_EXECUTOR))
            .flatMap(tick -> getDeliveriesObs())
            .doOnError(err -> Log.e("MPB", "Error retrieving messages" + err))
            .retry()
            .subscribe(this::parseDeliveries, Throwable::printStackTrace);
}

private Observable<List<Delivery>> getDeliveriesObs() {
    return RestClient.getInstance().getApiService().getDeliveries()
            .flatMap(Observable::from)
            .flatMap(this::getETAForDelivery)
            .toSortedList((d1, d2) -> {
                if (d1.getEta() == null) {
                    return -1;
                }
                if (d2.getEta() == null) {
                    return 1;
                }
                return d1.getEta().getDuration().getValue() > d2.getEta().getDuration().getValue() ? 1 : -1;
            });
}

Let's go step by step.

1. First we create an Observable that triggers every POLLING_INTERVAL time the method getDeliveriesObs() that will return the final list
2. We use retrofit to get an Observable of the call
3. We use flatMap to flattern the resut list and get in the next flatmap a Delivery item, one by one.
4. Then we get the estimated time of arrival set inside the Delivery object and return it
5. We sort the list to order by estimated time of arrival.
6. In case of error we print and retry so the interval does not stop
7. We subscribe finally to get the list sorted and with ETA inside, then we just return it or whatever you need to do with it.

It's working properly and it's quite nice, I'm starting to like rxjava :)

Answer 2

I haven't spent a lot of time with Java 8 lambdas, but here's an example of mapping each object to a different object, then getting a List<...> out at the other end in plain ol' Java 7:

List<Delivery> deliveries = ...;
Observable.from(deliveries).flatMap(new Func1<Delivery, Observable<ETA>>() {
    @Override
    public Observable<ETA> call(Delivery delivery) {
        // Convert delivery to ETA...
        return someEta;
    }
})
.toList().subscribe(new Action1<List<ETA>>() {
    @Override
    public void call(List<ETA> etas) {

    }
});

Of course, it'd be nice to take the Retrofit response (presumably an Observable<List<Delivery>>?) and just observe each of those. For that we ideally use something like flatten(), which doesn't appear to be coming to RxJava anytime soon.

To do that, you can instead do something like this (much nicer with lambdas). You'd replace Observable.from(deliveries) in the above example with the following:

apiService.getDeliveries().flatMap(new Func1<List<Delivery>, Observable<Delivery>>() {
    @Override
    public Observable<Delivery> call(List<Delivery> deliveries) {
        return Observable.from(deliveries);
    }
}).flatMap(...)